The present invention relates to a heat treatment for a compound semiconductor wafer, particularly including volatile elements.
Recently, the necessity of ultrahigh speed computers, ultrahigh frequency and optical communication, etc. has gradually increased in response to the tendency toward the information-communication society. Therefore, much investigation has been performed on compound semiconductors which have superior material properties, conventional semiconductor devices made of Silicon can not meet this necessity.
For example, when Gallium arsenide ingot is grown, defects such as dislocation, vacancy, interstitial atoms, and arsenide deposits are produced inside a crystal randomly so that the characteristics of a device which is fabricated by means of a wafer made of the ingot grown by a conventional LEC(Liquid Encapsulated Czochralski) method are not uniform, thereby degrading the reliability of the device. Thus, heat treatment is applied to the wafer to improve the defects and to provide uniform electrical characteristics.
FIG. 1 shows a schematic diagram illustrating a conventional method of heat treatment for a compound semiconductor wafer. First, protecting wafers 3 are contacted with both sides of a process wafer 1. The process and protecting wafers 1 and 3 are located in a quartz tube 7 together with excess reagent.
The quartz tube 7 is maintained in a vacuum state and sealed up. Subsequently the, inside of the quartz tube 7 is maintained at 800.degree..about.900.degree. C. for several minutes to hours by a heater 9 and subsequently cooled. During the heat treatment, dissociation of arsenide can easily occur since the volatile temperature of arsenic is much lower than that of gallium in the gallium arsenide wafer.
Thus, the tube 7 is in arsenic atmosphere by using the arsenide as excess reagent 5 to suppress the dissociation of arsenide from the wafer 1, and the protecting wafers 3 not only suppress the dissociation of arsenide from the wafer 1 but also provide arsenide to the thermal processing wafer 1. The vacuum of the tube 7 is maintained in order to prevent the oxidation and the contamination of the wafer 1 during the heat treatment.
The heat treatment is carried out in such a manner that the composition of arsenide in the wafer 1 is controlled by a gas pressure of the tube 7 maintained in arsenic atmosphere. Before the heat treatment, however, the arsenide deposit is randomly distributed on the wafer 1. Therefore, there is a problem in carrying out the dissociation and internal diffusion of arsenide uniformly on the surface of the wafer 1 during the heat treatment. Also, the surface becomes rough and defects such as vacancy are generated unevenly on the surface of the wafer 1.